Light-dimming is effected by adjusting the magnitude and/or the time of application of current to a light source(s). For example, control and dimming of a light source(s) using pulse width modulation (PWM) is well-known to those of ordinary skill in the relevant art. PWM-dimming works by varying the amount or length of time a switching device, such as a TRIAC, is in an ON state during a cycle relative to the total cycle time. This is referred to as the “duty cycle”. Typically, PWM-dimming supplies power (current) to the light source when an input voltage is above a threshold voltage (ON), and withholds power when the input voltage is below the threshold voltage (OFF). The longer the duty cycle, the relatively-brighter the light source(s), and the shorter the duty cycle, the relatively dimmer the light source(s).
Conventional, solid-state, PWM-dimming switches typically include a switching device, e.g., a TRIAC, and a firing capacitor. The charged time of the capacitor is delayed by the setting of the dimming device. Once the firing capacitor has reached a predetermined voltage, the capacitor discharges or “fires”, triggering the TRIAC. The TRIAC will continue to conduct electricity to the light source(s) until there is no current, which happen twice per cycle at each of the zero crossings of the AC current sine wave. Accordingly, by controlling the rate at which the capacitor is charged, one can control the firing time of the capacitor.
As an alternative to PWM-dimming directly from the TRIAC, relatively-large input and/or output storage capacitative devices can be used to capture and store charge (energy). Dimming can then be achieved by PWM-dimming at a higher frequency, to prevent flicker. Relatively large input and/or output storage capacitative devices, however, are expensive and consume space, hence, their use is undesirable in many applications.
However, as high-power, light-emitting diodes (LEDs) and LED clusters are used more and more in housing, commercial, and industrial applications to replace conventional light sources, e.g., incandescent light bulbs, the need to retrofit conventional, in situ light-dimming switches economically is desirable. However, conventional dimming switches cannot dim LEDs or LED clusters effectively, especially at low or relatively-low current levels, using just the phase angle of the input voltage.
Another problem with prior art PWM-dimming techniques includes flicker resulting from TRIAC jitter and noise. Analog-driven TRIACs experience jitter at their turn-on phase angle. The effect of the jitter is a function of current magnitude and the duty cycle.
U.S. Pat. No. 7,038,399 to Lys, et al. discloses an apparatus and method for powering solid-state, LED-based light sources using non-standard line voltage signals generated by a conventional AC dimming circuits. According to Lys, et al., a controller can be used to condition the AC signals generated by the AC dimming circuit, to drive the LED light sources.
Accordingly, it would be desirable to provide a power control system that adapts standard, phase-angle, wall-dimming switches for use with high-power LEDs and LED clusters similar to what has been disclosed by Lys, et al. but by using a sensed or average input voltage. Moreover, it would be desirable to provide a power control system that reduces flicker resulting from jitter using proportional current PWM-dimming.